A chainsaw-wielding robotic submarine is roving beneath Lois Lake in British Columbia, Canada. But it is not a prop left over from a sci-fi movie.

Rather, it is chopping down a forest that was left submerged decades ago when the valley was flooded by a hydroelectric dam. After it cuts the trees, they are floated to the surface, where they are dried out and sold to mills for use in furniture and construction, like any other lumber.

Trees left standing in flooded forests die, but they do not rot because the water keeps out oxygen. Worldwide, some 200 million trees are thought to be standing on the floor of hydropower reservoirs.

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